- A dwarf planet, discovered by Tombaugh in 1930. It was originally thought to be the ninth planet of the solar system, but in 2006 it was officially re-classified as a dwarf planet and part of the Kuiper Belt. Pluto’s exact mass is a matter of debate, although it seems to be about 0.25 per cent that of the Earth. It has not visited by space probes, so our knowledge of it is limited. Pluto’s largest moon Charon was discovered in 1978 and orbits 19,000km from Pluto. It is the largest satellite in the solar system relative to the body it orbits, being about 1300km across to Pluto’s 2300km. Observations of the interaction between Pluto and Charon have allowed our information on the dwarf planet to be enhanced. It has a density of about 2 gm/cc, which is consistent with a mix of rock and ice. Solid nitrogen appears to dominate its solid surface. Pluto’s other moons Nix and Hydra were discovered by the Hubble Space Telescope in 2005. Pluto’s orbit has an eccentricity of 0.25, so that it spends part of its time nearer to the Sun than Neptune, including the period from 1979 to 1999. Pluto’s mean distance from the Sun is 39AU, and it takes 248 years to orbit the Sun. Pluto’s minute size and its strange orbit formerly encouraged the idea that it is an escaped satellite of Neptune. Some satellites, including Neptune’s satellite Triton, are bigger than Pluto. A Hubble Space Telescope view in 1996 finally revealed that Pluto, which rotates on its axis in 6.4 days, has a surface with dark and bright spots and linear markings, whose nature is at present unknown. A ‘Pluto Express’ plan, for a mission which could visit Pluto after a 10–12 year spaceflight, was drawn up by US and Russian scientists but cancelled for budgetary reasons.